The present invention relates to telephone systems and more particularly to a supervisory circuit therefor.
There are three basic functions to be performed by a supervisory circuit serving an analog telephone subset, namely, (1) ring trip and ring present; (2) on-and-off hook detection and (3) dial pulse detection. A hit-timing provision is usually included in the supervisory circuit to obviate the likelihood of false responses due to line transients.
Ring present detection commonly uses an analog peak detector that monitors a voltage that is proportional to the AC (alternating current) ringing current. When this current is zero, the voltage is zero indicating that there is no ringing current. When the current rises to a value exceeding some threshold, the voltage at the output of the peak detector also crosses a threshold. If this remains for a sufficient period, a "ring present" indication is passed to the switching system from the supervisory circuit.
The ring trip detection is normally realized by using large capacitors to remove the large low frequency ringing signal so that the DC (direct current) current may be monitored by the off-hook detection circuit of the supervisory circuit. If the DC current is both at a threshold set on an analog comparator and it exists long enough, then a ring trip indication is passed to the switching system from the supervisory circuit.
On-and-off-hook detection is accomplished by creating a voltage proportional to the DC line current and comparing this voltage with a threshold set on an analog comparator. When the threshold is exceeded for a sufficient time, the off-hook indication is given by the supervisory circuit.
The thresholds are set to satisfy market requirements and the on-hook threshold is always lower than the off-hook threshold. This is called "hysteresis". It is usually accomplished by either changing the threshold voltage or providing positive feedback around the comparator to accomplish the same function.
Dial pulse detection is normally done using the on-and-off-hook detection circuit with the same threshold as required for hook detection.
Some of the disadvantages of the prior art supervisory circuits include (1) large value components are used to filter out large AC signals in order to detect DC signals; (2) the amplitude of reference voltages on analog amplitude comparators are difficult to make stable with time and temperature; and (3) peak detectors are difficult to construct in large scale integrated circuit environments.